This invention relates to the treatment of arsenic-containing sludge discharged after coagulation sedimentation treatment of liquid-waste for conversion into environmentally harmless constituents.
Methods for treating arsenic-containing waste water, as have been known, include the adsorption method, the ion-exchange method, the sulfide coagulation method, the hydroxide coprecipitation, and the like. Among these methods, the hydroxide coprecipitation method using calcium chloride, magnesium chloride, ferric chloride and the like is most common. FIG. 2 shows an example of flowcharts for this hydroxide coprecipitation method.
The arsenic-containing waster water 24 discharged from various types of plants flows into the reaction chamber 21. Arsenic is present in the waste water in the form of As.sup.3+ arsenic trioxide ion (AsO.sub.3.sup.3-) and As.sup.5+ arsenic acid ion (AsO.sub.4.sup.3-). When calcium chloride or ferrous chloride is added to this waste water, the reaction according to the following formula takes place to produce calcium arsenate, calcium arsenite or ferrous arsenate, which then precipitate. FIG. 2 illustrates the case in which hydrated lime is added as a representative additive. EQU 3Ca.sup.2+ +2AsO.sub.3.sup.3- .fwdarw.Ca.sub.3 (AsO.sub.3).sub.2(1) EQU 3Ca.sup.2+ +2AsO.sub.4.sup.3- .fwdarw.Ca.sub.3 (AsO.sub.4).sub.2(2) EQU Fe.sup.3+ +AsO.sub.3.sup.3- .fwdarw.FeAsO.sub.3 (3) EQU Fe.sup.3+ +AsO.sub.4.sup.3- .fwdarw.FeAsO.sub.4 (4)
Along with these reactions, calcium chloride and ferric chloride act as a coagulant, and the arsenic compounds formed in accordance with the foregoing equations (1), (2), (3) and (4) form particulate which gradually coarsen.
Next, this reaction fluid is led to a sedimentation tank 22 and the like to separate the solid matter, and the supernatant fluid is drained out of the system as a treated water 26, and the sediment of arsenic-containing sludge 27 is drawn from the bottom and processed by a dehydrator 23 to produce a dehydrated sludge cake 29. Further, a portion of is the sediment of the sludge 27 is returned to the reaction tank 21 as returned sludge 28.
However, the aforementioned conventional art has the following problems:
1) There has been no consistent treatment method established from waste treatment to sludge treatment. That is to say, the conventional art remained as a treatment process to extract arsenic from waste water into the sludge and failed to provide a process in succession to convert the arsenic formed then in the sludge to innocuous matter, posing many environmental problems.
2) If the sludge containing arsenic compounds produced in the treatment process of waste water is disposed and left in the environment after mere dehydrating and drying, arsenic components come out again with exposure to rainwater or underground water to become a new source of environmental pollution.